Chromium resists oxidation due to the formation of a thin, stable, and self-healing chromium oxide layer on its surface.
This passivating layer is key to chromium's corrosion resistance. Here's a more detailed explanation:
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Formation of Chromium Oxide: When chromium is exposed to oxygen, it reacts to form chromium(III) oxide (Cr₂O₃). This oxide layer is incredibly thin, typically just a few nanometers thick.
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Passivation: This Cr₂O₃ layer acts as a barrier, preventing further oxidation of the underlying chromium metal. This phenomenon is known as passivation. Essentially, the chromium oxide "passivates" the surface.
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Self-Healing Properties: If the chromium oxide layer is scratched or damaged, it will quickly reform in the presence of oxygen. This self-healing ability is crucial for maintaining corrosion resistance over time. This is because the chromium underneath the damaged area readily reacts with oxygen to replenish the protective layer.
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Stability and Impermeability: Chromium oxide is a very stable and tightly packed oxide, which makes it difficult for oxygen or other corrosive substances to diffuse through it to reach the underlying metal.
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Importance in Stainless Steel: Chromium is a vital alloying element in stainless steel. The addition of chromium (typically at least 10.5% by weight) allows stainless steel to form this protective chromium oxide layer, making it highly resistant to rust and corrosion. Without chromium, steel is susceptible to rapid oxidation (rusting).
In summary, chromium's resistance to oxidation stems from its ability to rapidly form a thin, adherent, impermeable, and self-healing layer of chromium oxide that protects the underlying metal from further corrosion.